The Real Reason Why Exercise Helps You Lose Weight (Hint: Brown Fat!)

You've read over and over again that eating less + exercise = weight
loss. But did you ever wonder exactly why working out helps you lose
weight -- or if it really does the trick at all (and doesn't just make you
hungrier)?

This exact connection between exercise and weight loss has been a bit of
a mystery even to the medical community, but a new study,
published in Nature and led by researchers at the Dana-Farber
Cancer Institute and Harvard Medical School, has helped clarify exactly
what's going on at a cellular level when you go for a run or hit the
elliptical. In a nutshell -- exercise causes your muscles to tell your
white fat to turn into brown fat.

What’s that? You didn’t know your muscles could talk to your fat cells? And what the heck is the difference between white fat and brown fat? We’ll explain.

Dr. Aaron M. Cypess, MD, PhD, a researcher and staff physician the Joslin Diabetes Center, broke it down for us: White fat is thusly named because animal fat is, indeed, white. (Cypess says human white fat is actually kind of yellowish — gross.) The main purpose of white fat, says Cypess, is to store an enormous amount of calories in a small amount of space. This, says Cypess, was very helpful when we roamed the plains and needed fat to keep from starving to death in between our scarce meals. Not as helpful these days, when we have unlimited access to calories — though a little bit of white fat is necessary to live. Having zero fat, as with anorexia, causes “all sorts of things to go haywire,” says Cypess, but the problem most of us face is having too much.

“The more white fat you get, the more likely you are to develop health problems like diabetes and heart disease,” Cypess says.

OK, so what about this mysterious brown fat? “Brown fat stores calories not for future use as an energy supply, but as fuel to burn off when it’s activated,” Cypess explains. He likens white fat to a huge gas tanker storing an enormous amount of fuel, while brown fat is a Lamborghini — it has gas in it because it needs it to run. Brown fat burns off calories specifically to generate heat, says Cypess, and it can do that in response to cold, or, it seems, in response to muscular activity (though further research is needed to prove this, says Cypess).

What’s exciting about the Dana Farber study is that, for the first time, researchers discovered that when you exercise, your muscles release a hormone the researchers named “irisin” (after Iris, the Greek messenger goddess). And what does irisin do? Cypess describes it like this: “It goes to the fat that is under the skin and induces it to grow more brown fat.”

So how can brown fat help you lose weight? “Eat a little bit less, and exercise more,” Cypess says, explaining that not only does exercise release irisin, but working out causes your muscles to work harder, which keeps them active after you stop exercising (meaning, you’re still burning calories later on).

The bottom line? Exercise increases your brown fat levels and brown fat burns calories rather than storing them — keeping you not just slimmer, but healthier.